Evidence that Enzyme Processivity Mediates Differential A Production by PS1 and PS2

The gamma-secretase complex cleaves the carboxy-terminal 99 residue (C99) fragment of the amyloid precursor protein (APP) to generate the amyloid-(A) peptide. The catalytic activity of this complex is mediated either by the presenilin-1 (PS1) or the presenilin-2 (PS2) subunit. In vitro and in vivo studies have demonstrated that PS1-containing complexes generate more total A beta product than PS2-containing complexes, indicating greater cleavage activity by PS1-containing gamma-secretase complexes at the APP gamma-site. However, it remains untested whether gamma-secretase cleavage at the APP epsilon-site, which precedes gamma-site cleavage and produces the physiologically active APP intracellular domain (AICD), follows the same rule. Using a novel Swedish APP-GVP substrate to facilitate the parallel detection of A beta and AICD products from PS1(-/-)/PS2(-/-) cells co-transfected with either PS1 or PS2, we observed that while PS1 generates more total A beta product than PS2, consistent with published reports, PS1 and PS2 unexpectedly generate equal amounts of AICD product. We also observed that PS1 and PS2 produce equivalent amounts of Notch intracellular domain (NICD), indicating equal cleavage activity at the Notch S3-site (the corollary of the APP epsilon-site). Our findings suggest that processivity differences between PS1 and PS2 underlie the differential production of A beta peptide. Taken together these findings offer novel insights into gamma-secretase biology and have important implications for therapeutically targeting gamma-secretase.